Network Working Group J. Klensin, WG Chair
Request For Comments: 1869 MCI
STD: 10 N. Freed, Editor
Obsoletes: 1651 Innosoft International, Inc.
Category: Standards Track M. Rose
Dover Beach Consulting, Inc.
E. Stefferud
Network Management Associates, Inc.
D. Crocker
Brandenburg Consulting
November 1995
SMTP Service Extensions
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
1. Abstract
This memo defines a framework for extending the SMTP service by
defining a means whereby a server SMTP can inform a client SMTP as to
the service extensions it supports. Extensions to the SMTP service
are registered with the IANA. This framework does not require
modification of existing SMTP clients or servers unless the features
of the service extensions are to be requested or provided.
2. Introduction
The Simple Mail Transfer Protocol (SMTP) [1] has provided a stable,
effective basis for the relay function of message transfer agents.
Although a decade old, SMTP has proven remarkably resilient.
Nevertheless, the need for a number of protocol extensions has become
evident. Rather than describing these extensions as separate and
haphazard entities, this document enhances SMTP in a straightforward
fashion that provides a framework in which all future extensions can
be built in a single consistent way.
3. Framework for SMTP Extensions
For the purpose of service extensions to SMTP, SMTP relays a mail
object containing an envelope and a content.
Klensin, et al Standards Track [Page 1]RFC 1869 SMTP Service Extensions November 1995
(1) The SMTP envelope is straightforward, and is sent as a
series of SMTP protocol units: it consists of an
originator address (to which error reports should be
directed); a delivery mode (e.g., deliver to recipient
mailboxes); and, one or more recipient addresses.
(2) The SMTP content is sent in the SMTP DATA protocol unit
and has two parts: the headers and the body. The
headers form a collection of field/value pairs
structured according to RFC 822 [2], whilst the body,
if structured, is defined according to MIME [3]. The
content is textual in nature, expressed using the US
ASCII repertoire (ANSI X3.4-1986). Although extensions
(such as MIME) may relax this restriction for the
content body, the content headers are always encoded
using the US ASCII repertoire. The algorithm defined in
[4] is used to represent header values outside the US
ASCII repertoire, whilst still encoding them using the
US ASCII repertoire.
Although SMTP is widely and robustly deployed, some parts of the
Internet community might wish to extend the SMTP service. This memo
defines a means whereby both an extended SMTP client and server may
recognize each other as such and the server can inform the client as
to the service extensions that it supports.
It must be emphasized that any extension to the SMTP service should
not be considered lightly. SMTP's strength comes primarily from its
simplicity. Experience with many protocols has shown that:
protocols with few options tend towards ubiquity, whilst
protocols with many options tend towards obscurity.
This means that each and every extension, regardless of its benefits,
must be carefully scrutinized with respect to its implementation,
deployment, and interoperability costs. In many cases, the cost of
extending the SMTP service will likely outweigh the benefit.
Given this environment, the framework for the extensions described in
this memo consists of:
(1) a new SMTP command (section 4)
(2) a registry of SMTP service extensions (section 5)
(3) additional parameters to the SMTP MAIL FROM and RCPT TO
commands (section 6).
Klensin, et al Standards Track [Page 2]RFC 1869 SMTP Service Extensions November 19954. The EHLO command
A client SMTP supporting SMTP service extensions should start an SMTP